fix incorrect prop passed to checkbox in docs examples (#12448)

prop should be `checked` instead of `value`

components/form/demo/dynamic-rule.md

@@ -75,7 +75,7 @@ class DynamicRule extends React.Component {
         </FormItem>
         <FormItem {...formTailLayout}>
           <Checkbox
-            value={this.state.checkNick}
+            checked={this.state.checkNick}
             onChange={this.handleChange}
           >
             Nickname is required